The Los Angeles Times said her condition was critical, quoting an unnamed source who said the actress was "in a lot of distress on the flight." The American actress has talked and written frequently about her years of drug addiction and mental illness.

The Los Angeles Fire Department (LAFD) did not refer to the actress by name but confirmed it had responded to an alert just after midday over "a patient on an inbound flight in cardiac arrest."

"LAFD firefighter paramedics were standing by and provided immediate advanced life support and aggressively treated and transported the patient to a local hospital," spokesman Erik Scott told AFP.

Fisher was catapulted to worldwide stardom as the rebel warrior Princess Leia in the original "Star Wars" trilogy, which has been a cultural phenomenon since the release of the films from 1977 to 1983.

Fisher was part of the star trio of the original Star Wars movie. Picture: Supplied

Steeped in Hollywood excess from an early age, she was the product of the four-year marriage of movie star Debbie Reynolds, best-known for her role in "Singin' In The Rain," and singer Eddie Fisher.

The relationship, and the happy home in Beverly Hills, came to an end when Fisher left Reynolds for her close friend, the actress Elizabeth Taylor.

Fisher is also known for her searingly honest semi-autobiographical novels, including her best-selling debut "Postcards from the Edge" which she turned into a film of the same name in 1990.

She has given various interviews over the years about her diagnosis of bipolar disorder and addiction to prescription drugs and cocaine, which she admitted using on the set of "The Empire Strikes Back" (1980).

She has also discussed being treated with electroconvulsive therapy, in which small electric currents are passed through the brain, to trigger brief seizures.

Writer and actress Anna Akana, who said she was on board Fisher's flight, described the midair drama in a series of tweets, voicing her "shock and sadness."

"Don't know how else to process this but Carrie Fisher stopped breathing on the flight home. Hope she's gonna be OK," she said.

"So many thanks to the United flight crew who jumped into action, and the awesome doctor and nurse passengers who helped Feel weird even tweeting about it but I JUST finished her book and was fangirling out over seeing her dog Gary in person."

Although she only recently admitted to a ‘hook-up’ with Harrison Ford she said her co-star was a ‘bad kisser’.Supplied

Fisher's famous "Star Wars" character features as part of the storyline to spin-off "Rogue One," which is currently riding high in box offices around the globe, although the actress is understood not to have been involved in the production.ft/dw

It was only in November that Fisher shocked Star Wars fans with the revelation that she hooked up with co-star Harrison Ford during the film, but that he was a "bad kisser" and no so hot in bed.

But Fisher, 60, says she let it slide because "he was really handsome."

In her tome The Princess Diarist, the actress recalls how she first met the then-34-year-old Ford on the London set of the 1977 blockbuster classic when she was just 19.

While Fisher has hinted in the past that the pair may have fooled around, she gets down and dirty in the book, revealing that he allegedly got her drunk and seduced her during filming, part of which took place in Tunisia.